This invention relates to polyphthalamide resin formulations having improved processing characteristics, more particularly to polyphthalamide resin formulations with reduced melt viscosities and to a method for lowering the melt viscosity of polyphthalamide resins.
Polyphthalamides generally exhibit a balance of thermal, strength and stiffness properties which make them suitable for many applications, and the resins may be particularly attractive for use where resistance to chemical and thermal attack is required. Polyphthalamide resins such as crystalline and semi-crystalline copolyphthalamides comprising at least about 50 mole percent aliphatic terephthalamide units are known for their particularly good thermal properties, and these resins have found acceptance for use where high temperature properties are desired. However, such polyphthalamides may have relatively high melting points, e.g. about 290.degree. C. or higher, and high melt viscosities, with degradation temperatures not greatly exceeding their melting points; accordingly, requirements for successful melt processing of these polyphthalamides are quite rigorous and complex. These high melt viscosity resins are particularly difficult to process into filaments. For example, conventional melt spinning operations place severe thermal stresses on high melt viscosity resins. The extruding of complex profiles and tube and pipe extrusion are also operations that may require placing high melt viscosity resins under thermal stress at high shear, particularly when using filled resins. In injection molding operations, successful filling of the mold, particularly where large or complex parts are molded, requires that the resin viscosity be reduced, which conventionally is accomplished by using heated molds and high stock and die temperatures. When fabricated in such operations resin degradation may become a serious problem, thus limiting the acceptability of polyphthalamides for many applications.
By contrast, most of the well-known aliphatic polyamides melt at lower temperatures, and the melt stability of these resins is quite adequate for thermal processing. For example, hexamethylene adipamide or nylon 66 melts at about 260.degree.-265.degree. C. and has a low melt viscosity, and thus is generally much more easily processed thermally than the high melting polyphthalamide resins. In some instances, aliphatic polyamides may even have unacceptably low melt viscosities and be difficult to thermally process, particularly where hot melt strength is needed such as in conducting melt spinning and extrusion operations. The use of additives or processing aids that will increase the melt viscosity of such resins to a level more optimum for melt processing has been proposed in the art.
Methods for lowering the melt viscosity of polyphthalamide resins are thus needed to improve the processability of these resins and to afford wider commercial acceptance, particularly where melt spinning of fiber is contemplated.
Polyamide fiber, including yarn and filament, is generally produced by melt spinning using processes well known and described in the art. In most such melt spinning operations, filtration means such as sand packs are conventionally employed to remove particulates including gel and other degradation products that would weaken the fiber. High melt viscosity may impede resin flow through such devices and cause substantial increase in back pressure, subjecting the resin to additional thermal stresses and further adding to the difficulty of spinning and extruding such resins.
The art of stabilizing resins against deterioration through exposure to thermal oxidative environments is well developed, and numerous additives are known for improving the thermal oxidative resistance of aliphatic polyamides, both during processing and while in use. However, because high temperature resins such as polyphthalamides require processing at temperatures very near the decomposition temperature, achieving adequate stabilization in these resins is more difficult than for aliphatic nylons.
The art thus continues to need better methods for avoiding thermal degradation of high temperature resins such as polyphthalamides. While a great many stabilizer packages have been disclosed in the art for these purposes, less effort has gone into reducing the mechanical and thermal stresses placed on the resin during thermal processing, particularly in melt spinning. A method for lowering the melt viscosity of polyphthalamide resins would provide an important advance in the art, allowing rapid processing of the resin and with lower shear, and may permit the use of lower processing temperatures.